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NST Chemistry > Shapes + Structures of Molecules I > Flashcards

Shapes + Structures of Molecules I Flashcards

Michaelmas

1
Q

acetyl fragment

A

CH3C(=O)-

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2
Q

toluene

A

benzene w methyl branch

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3
Q

chloroform

A

CHCl3

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4
Q

X ray crystallography is used to / shows

A

determine structure of crystals
bond lengths and angles
positions of atoms
packing of molecules

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5
Q

outline how x-ray crystallography works

A

beam of X-ray focused on crystal diffracts due to interactions with electrons
diffraction pattern analysed through electron density maps

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6
Q

why do H not show up on electron density maps?

A

H have very little e- density, do not significantly diffract x-rays in crystallography

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7
Q

disadvantages of x ray crystallography

A

good quality crystals needed
may be difficult to locate H’s
in solutions, crystals often have different structure

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8
Q

mass spectrometry is used to

A

find the weight of a molecule

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9
Q

mass spec method

A

sample vaporised, ionised
ions focused onto detector using mag/elec fields

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10
Q

two methods of ionising the sample for mass spec

A
  1. knock e- off by firing high energy e- at vapour
  2. electrospray: charged aerosol, ion attaches to molecule
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11
Q

IR spectroscopy is used to ___ by measuring _____

A

identify functional groups present in molecules
by measuring changes in molecular vibrational energy levels

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12
Q

molecules always possess what three types of energy?

A

translational, vibrational, rotational
all are quantised

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13
Q

transitions in what type of energy correspond with infrared radiation?

A

vibrational transitions

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14
Q

relationship between wavenumber and energy for transition

A

proportional as E=hv
higher wavenumber, more energy needed

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15
Q

frequency of a vibration depends on:

A
  1. mass 2. stiffness
    faster freq = stiffer spring, lighter mass
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16
Q

reduced mass

A

in a diatomic, both atoms vibrate
µ = m1m2 / m1+m2
for m1»m2, µ=m2 (esp for H)

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17
Q

fingerprint region

A

500-1500 (RHS)

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18
Q

double bonds region

A

1500-2000

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19
Q

triple bonds region

A

2000-2500

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20
Q

X-H single bonds region

A

2500+

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21
Q

there is only absorption of IR if…

A

there is a significant change in dipole moment, ie asymmetric stretch

which is why N2 in air doesn’t give rise to peaks, symmetric

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22
Q

define Raman spectroscopy

A

for homonuclear diatomics (symmetric, doesn’t absorb IR)
looks at frequency of light scattered by a sample
(heteronuclear diatomics are also Raman active)

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23
Q

N-H absorption

A

about 3300 cm-1

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24
Q

-NH2 group absorption feature, explain

A

usually two peaks (3300, 3400 ish)
due to the sym / asymm N-H stretches respectively

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25
why is the O-H stretch often broad?
H bonding between molecules possible, results in many slightly different strengths of O-H bonds
26
C triple bond N and C peaks
C-N > C-C due to greater dipole C-N 2250 C-C 2100-2250
27
C=C peak
1635-1690, weak
28
C=C peaks for benzene ring
multiple peaks between 1625-1450 medium-weak
29
-NO2 group IR spec
TWO peaks (sym/asym) 1350 (fingerprint), 1530
30
C=O peak
in KETONE, 1715 strengthen C=O bond --> higher weaken --> lower ("stiffer" spring = higher freq)
31
what causes strengthening / weakening of the C=O bond
EWG (eg. Cl) strengthen EDG (eg. NH2) weaken (dipole)
32
acyl chloride IR
1750-1830 cm-1 EWG strengthens C=O, higher than 1715
33
amide IR
1640-1690 cm-1 EDG weakens C=O, lower than 1715
34
carboxylic acid IR
1730 cm-1 weakly EWG
35
ester IR
1745 cm-1 weakly EWG
36
aldehyde IR
1730 cm-1 bc ketone (1715) alkyl groups weakly EDG compared to H of ald
37
acid anhydride IR
TWO stretches (Sym/asym) BUT sym 1820 > asym 1750 (vs asym>tym for NH2 / NO2)
38
how does conjugation affect the frequency of the C=O bond?
LOWERS base freq by 20-30 cm-1 as e- density spread over system
39
how does ring size affect freq of C=O
smaller ring, higher freq 35 cm-1 step
40
what does NMR measure?
transitions between energy levels of nuclear spin
41
how does NMR work?
radio waves of appropriate frequency cause transitions in energy levels of nuclei in a strong magnetic field
42
what is nuclear spin quantum number and how does it relate to energy levels?
I = 0, 1/2, 1, 3/2, ... nucleus spin I gives rise to 2I+1 different energy levels when placed in mag field, given by -ve to +ve m(I) in integer steps eg. I=1, m(I)= -1,0,1 I=3/2, m(I)= -3/2, -1/2, 1/2, 3/2
43
rules for I based on nuclei mass number
odd mass # = 1/2-integral spin odd #prot and neutron = integral even #prot and neutron = 0
44
energy difference between spin states depends on
strength of mag field, both supplied and local (e-dens, other nuclei) the nucleus itself
45
how does electron density impact the strength of the local magnetic field`
electrons move to oppose the mag field more e- = more shielding, lower freq e-neg atoms DESHIELD other atoms by withdrawing e-dens, higher freq
46
chemical shift scale NMR
ppm, relative to a reference compound shift = diff in freq / ref freq
47
standard reference compound used for NMR
TMS, tetramethylsilane inert, one signal, away from other peaks (to the right)
48
C NMR shift for sp3 C's
0-100 ppm more e-withdrawing (e-neg) groups attached --> higher freq
49
C NMR shift for sp2 C's
100-200 ppm more EWG, higher shift
50
C NMR shift for sp C triple bond C
70-80 ppm
51
what groups have shift around 200ppm?
ketones - just over aldehydes - just under
52
what groups have shift around 160-170ppm?
carboxylic acids, esters, acyl chlorides, amides, acid anhydrides note - LOWER shift than ald/ket despite more EWG attached! unexpected
53
how can quaternary C's be identified in C NMR?
much smaller line
54
how does coupling between atoms work?
if neighbouring atom also has spin states, a through bond interaction up reinforces mag field experienced by the C nuclei --> slightly higher freq down reduces --> slightly lower => doublet
55
why is C-C coupling not seen in C NMR?
low abundance of spin active C-13 tiny chance of C-13 - C-13 coupling lost in the noise could see if artificially enriched
56
when are splittings due to couplings of spin active nuclei NOT seen?
when the nuclei are EQUIVALENT (often symmetric) they still interact but the effects are not seen in splitting
57
how is a triplet splitting formed?
C couples to two other identical nuclei with spins
58
general rule splitting for coupling to multiple nuclei
coupling to n equivalent nuclei with spin I splits into 2nI+1 lines for I=1/2, pascals triangle
59
define equivalent nuclei (for coupling)
not only the same atom but also interchangeable positions! axial and equatorial are DIFFERENT for T-shape molecules
60
how is C NMR decoupled from protons?
by broadband proton decoupling - irradiating over a range of frequencies so all protons rapidly interconvert spin states, couplings to C average 0
61
what is the APT?
Attached Proton Test run on NMR spectrometer C with even# and odd# protons attached point up/down determine by solvent peak (CDCl3=0, even)
62
what are satellites in NMR?
some isotopes with spin that have low natural abundance this portion couples --> doublet (satellites), the rest appears as a singlet looks like a triplet (if the proportion is high enough) but really is a singlet w satellites
63
define vicinal coupling
the main difference between C NMR coupling and H NMR Coupling between protons easily seen separated by three bonds (vicinal) or more
64
H NMR shift scale
approx 0-14ppm (NOT the same scale as the CNMR ofc!)
65
vicinal coupling constant rough value
about 7 Hz
66
vicinal coupling constant value across C=C
cis about 8Hz trans about 15Hz
67
shift on protons on three-membered rings
unusually low, close to 0ppm
68
proton shift for H on C attached to double bonded C (C=C, C=O, Ar ring)
around 2.5
69
shift of H on C attached to N
around 2.5
70
shift on C attached to amide (N side)
3.5 ish C=O increases shift of H on C attached to N by 1
71
shift of H on C attached to O
around 3.5
72
shift of H on C attached to ester (O side)
around 4.5 C=O increases shift of H on C attached to O by 1
73
shift of H on C attached to Cl
3.7-4ish
74
H on C=C
4.5-6 ppm
75
shift of H on benzene ring
6-9ppm
76
shift of terminal H on aldehyde
9.5-10.5 ppm
77
shift of H on formate ester
8 ppm (a bit counterintuitive that this shift is lower than terminal H on aldehyde given there's another O)
78
feature of proton shifts of H attached to non-C atoms
they tend to be a lot broader than other signals but exact position is difficult to guess
79
D2O shake
identifying N-H and O-H protons by shaking NMR sample prepared in CDCl3 with D2O, which exchange these H for D signals of exchangeable protons disappear
80
define relaxation in spin states
rapid interconversion of spin states, most rapid in nuclei with spins greater than 1/2 also the reason why coupling to spins greater than 1/2 is not observed (eg. Cl, I=3/2)
81
Deuterium spin states
I=1 hence three spin states solvent CDCl3 split into triplet

NST Chemistry (16 decks)

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